Fluid bearing

ABSTRACT

In a fluid bearing comprising a bearing member adapted to rotatably support a shaft by means of a thin fluid film which is formed by supplying fluid under pressure into a small gap between the shaft and the bearing member and a control valve including a cylinder and a spool slidably received in the cylinder for controlling the pressure of the fluid, the bearing surface of the bearing member is provided with a plurality of control pockets and a plurality of detection pockets respectively surrounding the control pockets, the fluid is supplied under a predetermined pressure to the control pockets and the detection pockets, a pair of diametrically opposite detection pockets are connected to the opposite ends of the control valve for operating the spool, a pair of diametrically opposite control pockets respectively surrounded by the pair of diametrically opposite detection pockets are connected to the output of the control valve whereby upon creation of a pressure differential between the pair of detection pockets due to application of a load on the shaft, the fluid pressure in one of the pair of control pockets is increased whereas the fluid pressure in the other is decreased and wherein the control valve is balanced in accordance with the pressure differentials between the pair of detection pockets and between the pair of control pockets so that the pressure differentials are created between the pair of control pockets and the pair of detection pockets for restoring the shaft to the center of the bearing against the load. The spool is provided with a plurality of fluid passages inclined in the same direction with respect to the radius so that the fluid flowing through the passages creates a force tending to rotate the spool.

United States Patent Sept. 25, 1973 Unno et all,

[ FLUID BEARING [75] Inventors: Kunihiko Unno, Kariya; Kazuhiko Sugita,Anjo, both ofJapan Primary Examiner-Charles J. Myhre AssistantExaminer-Frank Susko ArromeyE. F. Wenderoth et al.

[57] 1 ABSTRACT In a fluid bearing comprising a bearing member adaptedto rotatably support a shaft by means of a thin fluid film which isformed by supplying fluid under pressure into a small gap between theshaft and the bearing member and a control valve including a cylinderand a spool slidably received in the cylinder for controlling thepressure of the fluid, the bearing surface of the bearing member isprovided with a plurality of control pockets and a plurality ofdetection pockets respectively surrounding the control pockets, thefluid is supplied under a predetermined pressure to the control pocketsand the detection pockets, a pair of diametrically opposite detectionpockets are connected to the opposite ends of the control valve foroperating the spool, a pair of diametrically opposite control pocketsrespectively surrounded by the pair of diametrically opposite detectionpockets are connected to the output of the control valve whereby uponcreation of a pressure differential between the pair of detectionpockets due to application of a load on the shaft, the fluid pressure inone of the pair of control pockets is increased whereas the fluidpressure in the other is decreased and wherein the control valve isbalanced in accordance with the pressure differentials between the pairof detection pockets and between the pair of control pockets so that thepressure differentials are created between the pair of control pocketsand the pair of detection pockets for restoring the shaft to the centerof the bearing against the load. The spool is provided with a pluralityof fluid passages inclined :in the same direction with respect to theradius so that the fluid flowing through the passages creates a forcetending to rotate the spool.

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ATTORNEY5 FLUID BEARING BACKGROUND OF THE INVENTION This inventionrelates to a fluid bearing for rotatably supporting a shaft through afluid film formed in a small gap between the bearing member and theshaft by sup plying fluid under pressure to pockets formed in thebearing member.

In a conventional fluid bearing, the bearing surface adapted rotatablysupport a shaft is provided with a plurality of pockets which aresurrounded by a plurality of lands divided by discharge grooves andfluid under pressure is supplied to these pockets through fixed ornonadgustable orifices. The pressurized fluid is discharged to thedischarge grooves through small gaps defined between the outerperipheral surface of the shaft and the lands to maintain a constantfluid pressure in respective pockets. Upon application of a load uponthe shaft, the shaft is displaced to vary the gaps between it and thelands so that the pressure in one pocket is increased whereas thepressure in the diameterically opposite pocket is decreased and thepressure differential between these pockets acts to support the load.The stiffness of this type of fluid bearing is not sufficiently highbecause constant pressure fluid is supplied to. the bearing throughfixed orifices so as to cause to vary the pressure in the pockets inresponse to the variation in the gap between the shaft and the lands.

To obviate this difficulty it has been proposed to provide a pluralityof independent detection pockets of the same number as the controlpockets adjacent thereto and communicated therewith through dischargegrooves. The pressure differential in opposed detection pockets is usedto operate a control spool or a slide valve to change the openings ofvariable orifices connected between a source of fluid pressure and thecontrol pockets so as to vary the pressure in the control pockets inaccordance with the pressure variation caused by the displacement of theshaft in the small gaps between the shaft and the lands and with thepressure variation caused by the adjustment of the variable orifice.With this arrangement it is possible to rapidly create a large pressuredifferential. between the opposed control pockets to resist thedisplacement of the shaft. Suchan arrangement is disclosed in U.S. Pat.No.

2,692,803, dated Oct. 26, 1954.

In the bearing disclosed in this patent, the pressure differential inthe detection pockets is not influenced by the pressure variationproduced by the adjustment of the variable orifices but influenced onlyby the pressure variation caused by the displacement of the shaft in thegap between the peripheral surface thereof and the lands with the resultthat the effective area subjected to the pressure variation caused bythe adjustment of the variable orifices becomes smaller than the totalbearing area, thus limiting the stiffness of the fluid bearing.

Further, a conventional control valve utilized to provide abovedescribed function comprises a cylinder and a spool disposed therein tobe slidable in the axial direction. However, the fluid supplied to thebearing and to the control valve usually consists of bearing oil havinga certain degree of viscosity, so that the axial movement of the spoolis affected by the viscosity or static friction of the bearing oil.Further as it is necessary to operate the control valve in response to arelatively small pressure differential in the bearing oil in the controlpocket and detection-pocket in order to improve the response of thecontrol valve the spool should move quickly and smoothly. But since theordinary bearing oil has some visocity and viscosity the viscosityvaries with ambient temperature, the movement of the spool becomesintermittently which is called as a stick slip phenomenon.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide an improved fluid bearing of high stiffness which can quicklybring back the shaft to the center of the hearing when a load is appliedto the shaft.

Another object of this invention is to provide an im proved fluidbearing of the type including a control pocket and a detection pocketwherein the shaft is quickly brought back to the center of the bearingwhen a load is applied to the shaft and wherein the pressure variationinthe detection pocket created by the control valve remains even afterthe shaft has been brought back to the center of the bearing against theload thus improving the load carrying capacity of the bearing.

Still another object of this invention is to provide an improved controlvalve for the fluid bearing of the type referred to above wherein theoperation of the control valve is not affected by the viscosity of thebearing oil.

According to this invention there is provided a fluid bearing of thetype comprising a bearing memberadapted to rotatably support a shaft bymeans of a thin fluid film which is formed by applying fluid underpressure into a small gap between the shaft and the bearing member and acontrol valve including a cylinder and a spool slidably received in thecylinder for controlling the pressure of the fluid, characterized inthat the bearing surface of the bearing member is provided with aplurality of control pockets and a plurality of detection pocketsrespectively surrounding the control pockets, that the fluid is suppliedunder a predetermined pressure to the control pockets and the detectionpockets, that a pair of diametrically opposite detection pockets areconnected to the opposite ends of the control valve for operating thespool, that a pair of diametrically opposite control pocketsrespectively surrounded by the pair of diametrically opposite detectionpockets are connected to the output of the control valve whereby uponcreation of a pressure differential between the pair of detectionpockets due to application of a load on the shaft, the fluid pressure inone of the pair of control pockets is increased whereas the fluidpressure in the other control pocket is decreased, and that the controlvalve is balnced in accordance with the pressure differentialsbetweenthe pair of detection pockets and between the pair of control pockets sothat pressure differentials are created between the pair of controlpockets and the pair of detection pockets for restoring the shaft to thecenter of the bearing against the load.

More particularly, the control valve comprises a cylinder, a hollowspool slidably received in the bore of the cylinder, and an annularchamlber provided for the bore to surround the spool, the spool beingprovided with a plurality of inclined passages which are contained in aplane perpendicular to the axis of the spool and inclined in the samedirection with respect to the radius thereof whereby the fluid flowingthrough the inclined passages creates a force for rotating the spool.Further, the spool is provided with a flange projecting into the annularchamber with small clearances therebetween so as to form variableorifices between the side walls of the annular chamber and the sidewalls of the flange. These orifices are communicated respectively with apair of diametrically opposite control pockets and a pair of detectionpockets encircling these control pockets and the degree of opening ofthe variable orifices is controlled in accordance with the axialmovement of the spool.

BRIEF DESCRIPTION OF THE DRAWINGS The invention can be more fullyunderstood from the following detailed description of a preferredembodiment taken in conjunction with the accompanying drawings in which:

FIG. 1 shows longitudinal sections of a fluid bearing and control valvesembodying the invention;

FIG. 2 shows a cross-section of the bearing taken along a line "-11 inFIG. 1;

FIG. 3 shows a cross-section of a control valve taken along a lineIIIIII in FIG. 1 and FIG. 4 is an enlarged developed view of a portionof the fluid bearing embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Fluid bearing shown in FIG. 1comprises a pair of bearing sleeves 11 and 12 which are securred inbearing 10 to oppose each other to define a chamber 18 therebetween.Within sleeves l1 and 12 is rotatably mounted a shaft 13 having anenlarged portion or flange 13a at the center which is freely received inchamber 18. Annular pockets 14 and 114 are formed on the opposed endsurfaces of sleeves l1 and 12, respectively. Pocket 14 and 114 arecommunicated with a source of pressurized fluid indicated by a referencecharacter P via fixed or non-adjustable orifices 16 and 116 respectivelywhereby the shaft 13 is prevented from being moved in the axialdirection by the static pressure of the fluid contained in pockets 14and 114.

As shown in FIG. 2, the inner bore of lefthand sleeve 11 is formed withindependent control pocket pairs 20, 21 and 22, 23 respectively in thedirection of load F and in the direction perpendicular thereto. Thesecontrol pockets to 23 are communicated with the source of pressurizedfluid P respectively through fixed orifices 24, 25, 26 and 27 torotatably support shaft 13 by the fluid under pressure supplied thereto.As shown in FIG. 4, a detection pocket 28 is formed to surround controlpocket 20, the detection pocket 28 being separated from control pocket20 by a rectangular land 32. Similarly, another control pockets 21, 22and 23 are also surrounded by detection pockets 29, 30 and 31 separatedtherefrom by rectangular lands 33 to 35, respectively. Consequently,control pockets 20 to 23 are communicated with detection pockes 28 to 31respectively through the gaps between rectangular lands 32 to 35 and theperipheral surface of shaft 13. As shown in FIG. 4, a supply port 36 anda control port are provided for detection pocket 28. Similarly,detection pockets 29 to 31 are provided with supply ports 37, 38 and 39(only port 37 is shown in FIG. 1) and these supply ports 36 to 39 arecommunicated with the source of pressurized fluid P via fixed orifices44 to 47 (orifices 46 and 47 are not shown). Further, detection pocket29 is provided with a control port 41 which corresponds to the controlport 40 of detection pocket 28. Axial discharge grooves 48, 49, 50 and51 are formed on the inner surface of the sleeve 11 between adjacentpairs of detection pockets 28 to 31, the opposite ends of dischargegrooves 48 to 51 being communicated with circumferential dischargegrooves 52 and 53 formed on the inner surface of sleeve 1 1. Thedischarge grooves 52 and 53 are communicated with a reservoir 150, asdiagrammatically shown in FIG. 1, so that the fluid under pressuresupplied to the control pockets 20 to 23 and to the detection pockets 28to 31 is drained to reservoir 150 through the gap between shaft 13 andsleeve 11 and through discharge grooves 52 and 53.

Like the lefthand sleeve 11, the righthand sleeve 12 is formed withcontrol pockets 120 to 123 (pockets 122 and 123 are not shown), fixedorifices 124 to 127 (orifices 126 and 127 are not shown) connectedbetween the control pockets 120 to 123 and the source of pressurizedfluid, detection pockets 128 to 131 (pockets 130 and 131 are not shown)respectively surrounding control pockets 120 to 123, fixed orifices 144to 147 (orifices 146 and 147 are not shown) connected between thedetection pockets 128 to 131 and the source of pressurized fluid anddischarge grooves 152 and 153.

Further, a control valve A is provided to be responsive to the pressuredifferential between the fluid pressures in detection pockets 28 and 29which are created by the displacement of shaft 13 under load F forcontrolling the pressure differential between control pockets 20 and 21and between detection pockets 28 and 29. Similar control valve 8 isprovided to be responsive to the pressure differential between detectionpockets 128 and 129 for controlling the pressure differential betweencontrol pockets 130 and 131 and that between detection pockets 128 and129. Since control valves A and B have the same construction, that ofcontrol valve A will be described in detail hereunder, and thecorresponding elements of both valves are designated by the samereference numerals. The cylinder of the control valve comprises splitclamped by means of bolts 61 to 64 (see FIG. 3) and contains a spool 65which is dis- I posed to be slidable in the axial direction androtatable. The opposite ends of a central bore 66 in the spool 65 aresealed by caps 77 and 78. Flanges 69 formed at the center of spool 65are received in an anular chamber 70 in cylinder 60 to be slidable alittle in the axial direction and rotatable. Left and right inletgrooves 71 and 72 are formed on the surface of the bore of the cylinder60 and spool 65 is provided with a plurality of inlet ports 73 and 74for communicating inlet grooves 71 and 72 with bore 66 inside spool 65.As shown in FIG. 3, a plurality of inclined passages 75 are provided forthe spool at a point intermediate flanges 69 for communicating annularchamber 70 with the axial bore 66 of spool 65, said passages 75 beingcontained in the same plane at right angles with respect to the axis ofthe spool and are inclined in the same direction with respect to theradius. Inlet grooves 71 and 72 are connected to the source ofpressurized fluid P and annular chamber 70 is communicated withreservoir through a discharge port 76. With this construction thepressurized fluid supplied to bore 66 through inlet grooves 71 and 72and inlet ports 73 and 74 is discharged to the reservoir throughinclined passages 75 and discharge port 76 so that the spool 65 isnormally rotated in the direction indicated by an arrow shown in FIG. 3.Such rotation overcomes the viscosity of the fluid contained in thecontrol valve and assures its quick and smooth axial movement.

Opposite ends of cylinder 60 of the control valve is closed by endplates 67 and 68 to form chambers 79 and 80 on the opposite ends ofspool 65. Lefthand chamber 79 (in the case of control valve B, chamber79 is located on the righthand side) is communicated with control port41 of detecting pocket 29 via a fixed orifice 81 whereas the righthandchamber 80 is communicated with control port 40 of detection pocket 28via a fixed orifice 82. On the opposite sides of annular chamber 70 areformed annular chambers 83 and 84. The effective area of annularchambers 83 and 84 is selected to be smaller than left-and righthandchambers 79 and 80. The gaps between side walls of annular chamber 70and the side walls of flanges 79 act as variable orifices 85 and 86.Lefthand annular chamber 83 (in the case of control valve B annularchamber 83 is located on the righthand side) is communicated withcontrol pocket whereas righthand annular chamber 84 (in the case ofcontrol valve B, annular chamber 84 is located on the lefthand side)with control pocket 21 with the result that a portion of the pressurizedfluid in control pockets 20 and 21 is discharged to reservoir 150 viaannular chambers 83 and 84, variable orifices 85 and 86, inclinedpassages 75 and discharge port 76. Accordingly, the pressure in thecontrol pockets 20 and 21 varies dependent upon the magnitude of the gapbe tween the shaft and sleeve 11 and the degree of opening of variableorifices 85 and 86, thus varying the pressure in detection pockets 28and 29 in accordance with the magnitude of the gap between shaft 13 andsleeve 11.

In the illustrated example, a grinding wheel 90 is mounted on thelefthand end of shaft 13 and a pulley 91 connected to a driving motor(not shown) through a belt is mounted on the righthand end.

The operation of the fluid bearing will be described with particularreference to the lefthand bearing sleeve.

When an upward load F is imposed upon grinding wheel 90 by a workpiece,not shown, the shaft 13 will be moved for a moment in the upwarddirection in the bearing, thus decreasing the upper gap while increasingthe lower gap between the shaft and the bearing surface. In response tothis small displacement of the shaft, the pressure in upper controlpocket 21 and detection pocket 29 increases whereas the pressure in thelower control pocket 20 and detection pocket 28 decreases. The pressuredifferential between detection pockets 28 and 29 moves spool 65 ofcontrol valve to the right as viewed in FIG. 1 to decrease the openingof variable orifice 86 whereas to increase that of variable orifice 85thus respectively decreasing and increasing the pressures in annularchambers 83 and 84. Consequently, the pressure in the upper controlpocket 21 is increased further whereas that in the lower control pocket20 is decreased further by the displacement of spool 65. The

variation of the pressure in control pockets 20 and 21 caused by thedisplacement of spool 65 results in the pressure rise in detectingpocket 29 whereas in the decrease of the pressure in detection pocket 28whereby to displace further spool 65. However, for the sake ofsimplicity of the effect of such small additional displacement will notbe discribed.

As above described the pressure in the upper control pocket 21 and upperdetection pocket 29 will be increased due to the displacements of shaft13 and spool 65 while the pressure in the lower control pocket 21 andlower detection pocket 28 will be decreased due to the displacements ofthe shaft and spool, thereby quickly restoring shaft 13 to the center ofthe bearing against load F As the shaft is brought back to the center ofthe bearing against the load, the pressure differentials between controlpockets 20 and 21 and between detection pockets 28 and 29 caused by thedisplacement of shaft 13 disappears. However, the pressure differentialscaused by the displacement of the spool of the control valve does notdisappear, thereby to maintain the shaft at the center of the bearingagainst load F. The pressure differencial between the pressures indetection pockets 29 and 28 acts on the opposite sides of spoole 65 ofcontrol valve A to hold spool 65 at a balanced position where a pressuredifferential is established between annular chambers 83 and 84corresponding to the first mentioned pressure differential.

As above described, in accordance with this invention, detection pocketsare provided to respectively surround a plurality of control pockets,the pressure differential between opposed detection pockets is appliedto the opposite ends of a control valve, and the pressure of the fluidsupplied to the control pockets is varied in accordance with thepressure differential acting upon the control valve. Accordingly, when aload is applied to the shaft, a force resisting the load is quicklyapplied to the bearing surface in accordance with the pressurevariations caused by the displacement of the shaft and by the operationof the control valve to bring back the shaft to the center of thebearing. Even after the shaft has been brought back to the center of thebearing, the pressure differential caused by the operation of thecontrol valve remains in the detection pockets, thus improving the loadcarrying capacity of the bearing.

Although the invention has been shown and described in terms of apreferred embodiment thereof it will be clear that many changes andmodifications may be made without departing from the true spirit andscope of the invention as defined in the appended claims.

We claim:

1. In a fluid bearing comprising a bearing member adapted to rotatablysupport a shaft by means of a thin fluid film which is formed bysupplying fluid under pressure into a small gap between said shaft andsaid bearing member and a control valve including a cylinder and a spoolslidably received in said cylinder for controlling the pressure of saidfluid, the improvement which comprises a plurality of control pocketsformed on the bearing surface of said bearing member, a plurality ofdetection pockets respectively surrounding said control pockets, meansfor supplying the fluid under a predetermined pressure for said controlpockets and said detection pockets, means for connecting a pair ofdiametrically opposite detection pockets with the opposite ends of saidcontrol valve for operating said spool and means for connecting a pairof diametrically opposite control pockets respectively surrounded bysaid pair of diametrically opposite detection pockets with the output ofsaid control valve whereby upon creation of a pressure differentialbetween said pair of detection pockets due to application of a load onsaid shaft, the fluid pressure in one of the pair of control pockets isincreased whereas the fluid pressure in the other control pocket isdecreased, said control valve being balanced in accordance with thepressure differentials between said pair of detection pockets and between said pair of control pockets so that upon application of a loadupon said shaft pressure differentials are created between said pair ofcontrol pockets and said pair of detection pockets for restoring saidshaft to the center of the bearing against said load.

2. The improvement according to claim 1 wherein said control valvecomprises: a cylinder, a hollow spool slidably received in the bore ofsaid cylinder, and an annular chamber provided for said bore to surroundsaid spool, said spool being provided with a plurality of inclinedpassages which are contained in a plane perpendicular to the axis ofsaid spool and inclined in the same direction with respect to the radiusthereof whereby the fluid flowing through said inclined passages createsa force for rotating said spool.

3. The improvement according to claim 2 wherein the opposite ends ofsaid hollow spool are closed to define a pair of fluid chambers betweenthe ends of said spool and the opposite ends of said cylinder, said pairof fluid chambers being subjected to a differential pressure to moveaxially said spool, fluid under pressure is supplied into said hollowspool and discharged through said inclined passages, and wherein saidspool is provided with a flange projecting into said annular chamberwith small clearances therebetween so as to form variable orificesbetween the side walls of said annular chamber and the side walls ofsaid flange, said inclined passages being provided through said flangeto open into said annular chamber,

4. A combination of a fluid bearing and a control valve, said bearingcomprising at least one bearing sleeve adapted to rotatably support ashaft, the bearing surface of said sleeve being provided with at leastone pair of diametrically opposite control pockets, means to supplyfluid under pressure to said control pockets through fixed orifices, atleast one pair of detection pockets respectively encircling said controlpockets, said control pockets and said detection pockets beingcommunicated with each other through small gaps between the periphery ofsaid shaft and the bearing surface of said bearing sleeve; and saidcontrol valve comprising a cylinder and a hollow spool slidably receivedin the bore of said cylinder, the opposite ends of said hollow spoolbeing closed to define fluid chambers between said opposite ends of saidhollow spool and the opposite ends of said cylinder, means tocommunicate said fluid chambers with said diametrically oppositedetection pockets so as to apply a pressure differential between saiddetection pockets on the opposite ends of said spool, a flange formed atthe central outer portion of said spool, an annular chamber provided forsaid bore for loosely receiving said flange, means to supply fluid underpressure into said hollow spool, said flange being provided with aplurality of inclined passages which are contained in a common planeperpendicular to the axis of said spool and are inclined with respect tothe radius so that said fluid under pressure flows through said inclinedpassages into said annular chamber to create a force tending to rotatesaid spool, the opposite sides of said flange being spaced a little fromthe opposite side walls of said annular chamber to define a pair oforifices therebetween, the degree of opening of said orifices beingvaried by the axial movement of said spool, and means for connectingsaid pair of orifices to said pair of control pockets.

1. In a fluid bearing comprising a bearing member adapted to rotatablysupport a shaft by means of a thin fluid film which is formed bysupplying fluid under pressure into a small gap between said shaft andsaid bearing member and a control valve including a cylinder and a spoolslidably received in said cylinder for controlling the pressure of saidfluid, the improvement which comprises a plurality of control pocketsformed on the bearing surface of said bearing member, a plurality ofdetection pockets respectively surrounding said control pockets, meansfor supplying the fluid under a predetermined pressure for said controlpockets and said detection pockets, means for connecting a pair ofdiametrically opposite detection pockets with the opposite ends of saidcontrol valve for operating said spool and means for connecting a pairof diametrically opposite control pockets respectively surrounded bysaid pair of diametrically opposite detection pockets with the output ofsaid control valve whereby upon creation of a pressure differentialbetween said pair of detection pockets due to application of a load onsaid shaft, the fluid pressure in one of the pair of control pockets isincreased whereas the fluid pressure in the other control pocket isdecreased, said control valve being balanced in accordance with thepressure differentials between said pair of detection pockets andbetween said pair of control pockets so that upon application of a loadupon said shaft pressure differentials are created between said pair ofcontrol pockets and said pair of detection pockets for restoring saidshaft to the center of the bearing against said load.
 2. The improvementaccording to claim 1 wherein said control valve comprises: a cylinder, ahollow spool slidably received in the bore of said cylinder, and anannular chamber provided for said bore to surround said spool, saidspool being provided with a plurality of inclined passages which arecontained in a plane perpendicular to the axis of said spool andinclined in the same direction with respect to the radius thereofwhereby the fluid flowing through said inclined passages creates a forcefor rotating said spool.
 3. The improvement according to claim 2 whereinthe opposite ends of said hollow spool are closed to define a pair offluid chambers between the ends of said spool and the opposite enDs ofsaid cylinder, said pair of fluid chambers being subjected to adifferential pressure to move axially said spool, fluid under pressureis supplied into said hollow spool and discharged through said inclinedpassages, and wherein said spool is provided with a flange projectinginto said annular chamber with small clearances therebetween so as toform variable orifices between the side walls of said annular chamberand the side walls of said flange, said inclined passages being providedthrough said flange to open into said annular chamber.
 4. A combinationof a fluid bearing and a control valve, said bearing comprising at leastone bearing sleeve adapted to rotatably support a shaft, the bearingsurface of said sleeve being provided with at least one pair ofdiametrically opposite control pockets, means to supply fluid underpressure to said control pockets through fixed orifices, at least onepair of detection pockets respectively encircling said control pockets,said control pockets and said detection pockets being communicated witheach other through small gaps between the periphery of said shaft andthe bearing surface of said bearing sleeve; and said control valvecomprising a cylinder and a hollow spool slidably received in the boreof said cylinder, the opposite ends of said hollow spool being closed todefine fluid chambers between said opposite ends of said hollow spooland the opposite ends of said cylinder, means to communicate said fluidchambers with said diametrically opposite detection pockets so as toapply a pressure differential between said detection pockets on theopposite ends of said spool, a flange formed at the central outerportion of said spool, an annular chamber provided for said bore forloosely receiving said flange, means to supply fluid under pressure intosaid hollow spool, said flange being provided with a plurality ofinclined passages which are contained in a common plane perpendicular tothe axis of said spool and are inclined with respect to the radius sothat said fluid under pressure flows through said inclined passages intosaid annular chamber to create a force tending to rotate said spool, theopposite sides of said flange being spaced a little from the oppositeside walls of said annular chamber to define a pair of orificestherebetween, the degree of opening of said orifices being varied by theaxial movement of said spool, and means for connecting said pair oforifices to said pair of control pockets.